Abstract

The introgression of genes from exotic species or populations into gene pools of native species is a widespread
concern in agricultural systems. This is also an issue of increasing importance in forest systems as
there has been a dramatic expansion of tree plantations, which have now reached 180 million ha globally.
This has recently occurred in Australia with eucalypts. To help assess the risk of genetic pollution, we assess
the pattern of realised pollen dispersal from exotic Eucalyptus nitens plantations into native E. ovata forest
in Tasmania. We assessed the frequency of F1 hybrids in open-pollinated seed collected from native E.
ovata trees located at varying distance from three exotic E. nitens plantations in Tasmania. Over 119,000
seedlings were screened for morphological markers diagnostic of each species and the F1 hybrid. F1 hybridisation
averaged 7.2% within 100 m of the exotic E. nitens, with one native tree reaching 56%, but
diminished to 0.7% by 200–300 m and continued at this low level to the limits of the sampling at 1.6 km.
The decay in the percentage of interspecific F1 hybridisation with distance followed a power function with a
negative exponent (%F1 = 91.435distance)0.789; R2 ¼ 0.84). Eucalyptus nitens is exclusively pollinated by
small insects (smaller than honeybees), which the study shows can disperse pollen over 1.6 km. However,
the restriction of most exotic F1 hybridisation to within 200 m of exotic plantations presents clear
opportunities to manage the genetic impacts of plantations on native forests.